The crystallization kinetics of 50nm thick Ge2Sb2Te5 films, deposited by sputtering on a thermally grown SiO2 layer, has been investigated by transmission electron microscopy (TEM) and time resolved reflectivity (TRR) measurements during isothermal annealing in the range between 120 C and 130 C. Three different amorphous states have been analyzed: as deposited, primed and melt quenched. At the same temperature, the primed amorphous sample exhibits a higher nucleation rate whereas the melt quenched film shows an enhancement of both nucleation and growth rates with respect to the as deposited film. This behavior can be explained taking into account the effects of laser irradiation on the local order and on the population of subcritical nuclei in the amorphous phase.